تحقیق Ram

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Ram Guide

What is random access memory (RAM)?

Random access memory

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Random access memory or RAM is a type of computer storage whose contents can be accessed in any order. This is in contrast to sequential memory devices such as magnetic tapes, discs and drums, in which the mechanical movement of the storage medium forces the computer to access data in a fixed order. It is usually implied that RAM can be both written to and read from, in contrast to read-only memory or ROM.

Overview

Computers use RAM to hold the program code and data during execution. In the first electronic computers, RAM was built from vacuum tubes, and later magnetic cores. The term "core" is still used by some programmers to describe the RAM at the heart of a computer.

Many types of RAM are volatile, which means that unlike some other forms of computer storage such as disk storage and tape storage, they lose their data when the computer is powered down.

Throughout the history of computing, a variety of technologies have been used for RAM, and usually more than one in the same computer, with high-memories constructed out of the same technology as the logic, and slower, cheaper technologies used for bulk storage.

Some early computers used mercury delay lines, in which a series of acoustic pulses were sent along a tube filled with mercury. When the pulse reached the end of the tube, the circuitry detected whether the pulse represented a binary 1 or 0 and caused the oscillator at the beginning of the line to repeat the pulse. Other early computers stored RAM on high-speed "magnetic drums".

Later designs used arrays of small ferrite electromagnets, known as core memory.

Modern RAM generally stores a bit of data as either a charge in a capacitor, as in dynamic RAM, or the state of a flip-flop, as in static RAM.

Common types of RAM

SRAM or Static RAM

DRAM or Dynamic RAM

Fast Page Mode DRAM

EDO RAM or Extended Data Out DRAM

SDRAM or Synchronous DRAM

DDR SDRAM or Double Data Rate Synchronous DRAM

RDRAM or Rambus DRAM

Not so common types of RAM

Dual-ported RAM

Video RAM, a dual-port memory with one random access port and one sequential access port.

RAM packaging

Semiconductor RAM is produced as integrated circuits (ICs). RAM ICs are often assembled into plug-in modules. Some standard module types are:

single-in-line memory module (SIMM)

dual-in-line memory module (DIMM)

Write-Only Memory

In 1972 some engineers at the Signetics Corporation published a spoof data sheet for a write-only memory. This was a type of RAM with no read facility and therefore no outputs. The full title was Fully Encoded, 9046×N, Random Access Write-Only Memory and the part number was 25120.

Introduction

Three years ago, there wasn't much to say about sytem RAM. Almost all PCs came with fast page mode (FPM) DRAM, which ran at speeds between 100ns and 80ns. However, escalating CPU and motherboard bus speeds outstripped the ability of FPM DRAM to deliver data in a timely manner. Nowadays there are a lot of different memory designs.

Due to cost considerations, all but the very high-end (and very expensive) computers have utilized DRAM for main memory. Originally, these were asynchronous, single-bank designs because the processors were relatively slow. Most recently, synchronous interfaces have been produced with many advanced features. Though these high-performance DRAMs have been available for only a few years, it is apparent that they will soon be replaced by at least one of the protocol-based designs, such as SyncLink or the DRDRAM design from Rambus, Inc. and Intel.

Basic DRAM Operation

A DRAM memory array can be thought of as a table of cells. These cells are comprised of capacitors, and contain one or more 'bits' of data, depending upon the chip configuration. This table is addressed via row and column decoders, which in turn receive their signals from the RAS and CAS clock generators. In order to minimize the package size, the row and column addresses are multiplexed into row and column address buffers. For example, if there are 11 address lines, there will be 11 row and 11 column address buffers. Access transistors called 'sense amps' are connected to the each column and provide the read and restore operations of the chip. Since the cells are capacitors that discharge for each read operation, the sense amp must restore the data before the end of the access cycle.

The capacitors used for data cells tend to bleed off their charge, and therefore require a periodic refresh cycle or data will be lost. A refresh controller determines the time between refresh cycles, and a refresh counter ensures that the entire array (all rows) are refreshed. Of course, this means that some cycles are used for refresh operations, and has some impact on performance.

A typical memory access would occur as follows. First, the row address bits are placed onto the address pins. After a period of time the RAS\ signal falls, which activates the sense amps and causes the row address to be latched into the row address buffer. When the RAS\ signal stabilizes, the selected row is transferred onto the sense amps. Next, the column